Monolithic microwave integrated circuits (MMICs) are becoming more and more prevalent in the microwave industry due to their small size and weight, enhanced performance, increased reliability, and low cost. Circuits which in the past consisted of many discrete elements connected together in some complex configuration can now be put on a single chip of considerably smaller dimensions with no significant reduction in performance. Before these MMIC chips can be marketed and used in systems, however, they have to be packaged. These packages allow users to simply drop the MMICs into predesigned circuits without the need for intricate wire bonding to the chip itself. They also protect the chip from potentially damaging environmental conditions, such as ambient moisture level and salt atmosphere, and from contamination. A problem which arises with MMIC packaging is that the packages themselves can severely limit the performance of the chip. Therefore, it is a priority that any performance limiting aspects of the packaging be discovered and adequately dealt with.
The microwave performance limitation of MMIC packages is known to be caused in large part by poor feedthrough designs. A feedthrough is simply a port through which microwave energy is transmitted into or out of a microwave package. In more general terms, it is a device for transmitting electromagnetic energy through an aperture in a conductive surface. It is very important to the overall performance of the MMIC that the RF feedthrough provide low insertion loss, high return loss, and no resonances over the operating frequency range of the MMIC chip. As MMIC chips achieve higher and higher frequency capabilities, the performance of the feedthroughs becomes more and more critical.
The basic electrical requirements of a feedthrough are a return loss greater than 15 dB and an insertion loss of less than 0.02 dB/GHz, across the entire operational frequency range of the chip, when matched into a 50 .OMEGA. line. In employing the known prior art, it is difficult to achieve such performance goals over the entire 1-20 GHz frequency range.
FIGS. 1A and 1B show the feedthrough structure of a typical prior art microwave package. This structure employs multilayer, ceramic, thick film technology using 20 mil thick, 94% alumina substrates, and tungsten, nickel, and gold metallization. Referring to the figure, the device includes ceramic substrates 50, 58, 60, and 64. Deposited upon the upper surface of substrate 50 is conductive strip 56 and conductive ground pads 52 and 54. The conductive strip extends from edge 72 of substrate 50 to edge 70 of said substrate, and is centered on said substrate between edges 74 and 76. The conductive strip has a section of reduced width situated between two sections of equal width.
Conductive ground pads 52 and 54 are situated one on each side of said strip, and equidistant from said strip, and extend from edge 72 to a point even with the far end of the section of reduced width of said conductive strip. Substrate 58 is situated on top of substrate 50 and covers the section of reduced width of the conductive strip, a small portion of each of the sections of equal width of the conductive strip, and a considerable portion of each of the two conductive ground pads. Substrate 60 is situated on top of substrate 58 and fully covers it. Substrate 60 has conductive ground plane 62 deposited on its upper surface. Substrate 64 is situated below substrate 50 and fully covers its lower surface. Substrate 64 has conductive ground plane 78 deposited on and fully covering its lower surface.
A plurality of via holes 66 are made through the planar substrates for the purpose of creating electrical continuity between conductive ground plane 62, conductive ground plane 78, and conductive ground pads 52 and 54. These holes have their inner surfaces metallized to provide such continuity.
Each of the two sections of equal width of conductive strip 56, in conjunction with substrates 50 and 64, and conductive ground plane 78, acts as the center conductor of a microstrip transmission line section in its respective region. The section of reduced width of the conductive strip, in conjunction with substrates 50, 58, 60, and 64, and conductive ground planes 62 and 78, acts as the center conductor of a stripline transmission line section in its region. Conductive ground pads 52 and 54 act to suppress radiated power by providing a low impedance ground path.
When implemented to transmit microwave energy through the wall of a MMIC package, the device will be situated in an aperture in said wall so that the periphery of said aperture surrounds the device across the top of substrate 60, down each side of said device, and across the bottom of substrate 64. The length of substrate 60 will be equal to the thickness of the wall. When operating, a microwave signal will be applied to conductive strip 56 at either edge 70 or edge 72 and this signal will be transmitted to the opposite edge, through the three separate transmission media, from which it will then be transmitted away from the device by an external means.
When this feedthrough structure is tested over the 1-20 GHz frequency range, a resonance is found in both the forward transmission (S.sub.21) and forward reflection (S.sub.11) plots at about 13.9 GHz. This resonance was found to be caused by a parasitic inductance created by the tungsten filled via holes connecting the upper ground plane to the lower ground plane. Because of this finite parasitic inductance, the upper ground plane is not adequately grounded and this causes the excitation of an unbalanced TEM mode when certain frequencies are present. A simple equivalent circuit representation of the structure can be seen in FIG. 2. The parasitic inductances of the via holes and the capacitance between the center conductor of the stripline and the upper ground plane combine to form a band pass filter which shunts the flow of microwave energy on the center conductor to ground at the filter's resonant frequency. To alleviate this problem, it is necessary to move the resonant frequency of this parasitically created filter out of the operational frequency range of the MMIC chip. The subject invention accomplishes this by employing a unique microwave structure which reduces the coupling between the upper ground plane and the center conductor.